1. Field of the Invention
The present invention relates to a moving picture coding system.
2. Description of the Background Art
A moving picture coding system is able to have higher coding efficiency with a B picture (a predicted image that may be based on the motion vectors) than the coding efficiency when using only P pictures (a predicted image based on one motion vector).
For the B picture, the block prediction method for a direct mode involves calculating a forward motion vector and a backward motion vector as scaled versions of a motion vector of a co-located block in a backward reference picture for direct mode, to then obtain two distinct motion-compensated blocks using the forward and backward motion vectors, respectively. A predicted block is then obtained by averaging the two motion-compensated blocks.
The block prediction method for the direct mode as described above will be described in more detail with reference to FIG. 1.
FIG. 1 is a view showing a picture pattern for describing the block prediction method for the direct mode according to the conventional art. The picture pattern comprises an I-picture (not shown) coded using prediction only from decoded samples within the same picture (e.g., intra prediction), P pictures P1, P4, and P7 coded by inter prediction using at most one motion vector from previously-decoded reference pictures, and B-pictures B2, B3, B5 and B6 coded by two inter prediction blocks from previously-decoded reference pictures.
Also, parameters shown in FIG. 1 will first be described first for the sake of convenience. TRD represents a temporal distance between a forward reference picture for direct mode (P1) and a backward reference picture for direct mode (P7), TRB represents a temporal distance between the forward reference picture for direct mode (P1) and a current B picture (B5), MV represents a motion vector of a co-located block in the backward reference picture for direct mode (P7), MVf represents a forward motion vector of direct mode pointing to the forward reference picture for direct mode, and MVb represents a backward motion vector of direct mode pointing to the backward reference picture for direct mode. Herein, the forward reference picture for direct mode is a reference picture pointed by the motion vector of the co-located block in the backward reference picture for direct mode.
The block prediction method for direct mode will be described using the above parameters as follows.
First, the forward motion vector of direct mode (MVf) is obtained from a motion vector (MV) of a co-located block Bs in the backward reference picture for direct mode (P7) by applying following equation (1).
                              MV          f                =                                            TR              B                        ×            MV                                TR            D                                              (        1        )            
In addition, the backward motion vector of direct mode (MVb) is obtained from a motion vector (MV) of the co-located block Bs in the backward reference picture for direct mode (P7) by applying following equation (2).
                              MV          b                =                              (                                          TR                B                            -                              TR                D                                      )                    ×                      MV                          TR              D                                                          (        2        )            
Therefore, blocks Bf and Bb are motion-compensated using the motion vectors MVf and MVb calculated from equations (1) and (2), and after that, the two blocks are averaged to get a prediction value Bc′ of a current block Bc in the B picture as following equation (3).
                              B          c          ′                =                                            B              f                        +                          B              b                                2                                    (        3        )            
However, according to the block prediction method for the direct mode of the conventional art, the forward motion vector of direct mode is obtained from the motion vector of the co-located block in the backward reference picture for direct mode, and therefore, the obtained value is just an approximated value, not a precise motion vector of the current block of the B picture.
Also, according to the block prediction method for direct mode of the conventional art, even though the reference picture temporally close to the B picture has higher similarity with the B picture, the block prediction is made using the average of two distinct motion-compensated blocks without considering temporal distance between the reference pictures. Therefore, the accuracy of predicted block is lowered.
Especially, in a sequence having a fading scene, since brightness of continuous B pictures can be gradually darkened or gradually lightened, the prediction value obtained by simply averaging two motion-compensated blocks can differ significantly from the original value, and thereby the coding efficiency of the entire system is greatly lowered.